excretion

• excretion

  process by which toxic waste products of metabolism + substances in excess are removed from the body
• organs involved in excretion

  - liver

  - kidneys

  - lungs

  - skin
• hepatic artery

  carries oxygenated blood to the liver

  • provides oxygen for aerobic respiration - fuels metabolic activity
• hepatic portal vein

  carries blood from the digestive system

  • allows liver to absorb nutrients from the small intestine
• hepatic vein

  carries deoxygenated blood away from the liver and back towards the heart
• gall bladder

  directly connected to liver, stores bile

  • releases bile into duodenum via bile duct
• lobules

  arrangement of hepatocytes

  • supplied with blood from hepatic artery + vein
  • blood mixes within lobules inside sinusoids - exchanges substances with hepatocytes
• glycogenesis

  the conversion of glucose into glycogen
• glycogenesis

  • insulin triggers this process
  • glycogen synthesis removes glucose from bloodstream, decreasing conc. in bloodstream to within a normal range
  • glycogen is stored within hepatocytes - compact storage molecule
• urea formation

  protein from diet is broken down into amino acids, which is transported to the liver in the blood by the the hpv

  • excess amino acids are processed inside hepatocytes during deamination + the ornithine cycle
• deamination

  $NH_2$ is removed from each amino acid, along with an extra H$^+$ atom

  • combine to form ammonia - $NH_3$
  • remaining amino acid is a keto acid which can be used in respiration, converted to glucose or glycogen for storage
• the ornithine cycle

  ammonia is very soluble + highly toxic - urea is less toxic

  • $2NH_3\ +$$CO_2\ \rightarrow\ CO\left(NH_2\right)_2\ +$$H_2O$
  • ammonia + carbon dioxide = ammonia = water
  • urea diffuses through bilayer of hepatocytes and is transported to kidneys to be excreted
• detoxification

  the breakdown of unneeded/toxic substances
• detoxification of alcohol

  • ethanol is absorbed into the stomach and transported in the blood to the liver
  • in hepatocytes, alcohol dehydrogenase converts ethanol into ethanal for respiration
• detoxification of alcohol

  if continuous can cause problems:

  • metabolism of alcohol generates ATP, hepatocytes store fat instead of metabolising, causing fatty liver
  • stored fat reduces hepatocytes ability to do other functions and can lead to cirrhosis
• kidney functions

  • osmoregulatory organ
  • excretory organ
• renal artery

  carries oxygenated blood containing urea + salts to kidneys
• renal vein

  carries deoxygenated blood away from kidneys
• cortex

  • glomerulus + bowmans capsule
  • proximal convoluted tubule
  • distal convoluted tubule
• medulla

  • loop of henle
  • collecting duct
• nephron

  functioning unit of the kidneys

  • afferent arteriole supplies glomerulus with blood from renal artery
  • glomerulus capillaries rejoin to form efferent arteriole
  • blood throws through capillary network that runs alongside nephron
  • blood flows into renal vein
• urine formation

  ultrafiltration $\rightarrow$ small molecules filtered from blood into Bowman's capsule, forming filtrate

  selective reabsorption $\rightarrow$ useful molecules taken back from filtrate and returned to blood
• urine formation

  • ultrafiltration → small molecules filtered from blood into Bowman's capsule, forming filtrate
  • selective reabsorption → useful molecules taken back from filtrate and returned to blood
  • remaining filtrate forms urine
  • kidneys $\rightarrow$ ureter $\rightarrow$ bladder
• ultrafiltration

  • arterioles branch off renal artery and lead to nephron and form glomerulus in bowman's capsule
  • afferent arteriole (enters glomerulus) is wider than the efferent arteriole (leaves glomerulus) - high blood pressure within glomerulus
  • causes smaller molecules in the blood to be forced out of capillaries of the glomerulus, into bowman's capsule forming filtrate
• glomerular filtrate

  • amino acids
  • water
  • glucose
  • urea
  • inorganic ions
• features aiding ultrafiltration

  two cell layers + basement membrane between glomerular capillaries and bowman's capsule

  • endothelium of capillary - small gaps between endothelial cells allow small molecules to pass through
  • basement membrane - mesh of collagen + glycoproteins - small molecules can pass through holes in mesh
  • epithelium of bowman's capsule - many projections (podocytes) - gaps so small molecules can pass through
• proximal convoluted tubule

  most water + salts reabsorbed here

  • composed of singular layer of epithelial cells
  • many microvilli on luminal membrane increase SA for reabsorption
  • many co-transporter proteins in luminal membrane that transport specific solute
  • many mitochondria provide energy for NA/K pumps
  • cells tightly packed together so no fluid can pass between cells - reabsorbed substances must pass through cells
• reabsorption mechanisms

  • active transport moves $Na^+$ from PCT into tissue, $Cl^-$ follows in by diffusion as electrical gradient created
• selective reabsorption

  • Na/K pumps use ATP from mitochondria to move $Na^+$out of PCT cells into the blood, where they're carried away - lowering conc
  • $Na^+$ diffuse down conc gradient passively into epithelial cells from filtrate
  • this occurs via co-transporter proteins that bring in glucose + amino acids at the same time
  • once inside epithelial cells, solutes diffuse down conc gradient into blood
• where does selective reabsorption occur?

  in the proximal convoluted tubule, via cotransport proteins
• reabsorption of water + salts

  • $Na^+$/$Cl^-$ pumped out of filtrate in ascending limb into surrounding medulla, lowering $\Psi$
  • ascending limb - impermeable to water, water cannot leave loop by osmosis, $\Psi$ increases as water rises to back to cortex solutes removed
  • descending limb - permeable to water, water moves out by osmosis due to low $\Psi$ of medulla - $\Psi$ of filtrate decreases due to water loss
  • low $\Psi$ in medulla created by ascending limb enables reabsorption of water from collecting duct - water + ions that leave loop go into nearby capillaries
• osmoregulation

  control of water potential of body fluids

  • osmoreceptors in hypothalamus monitor $\Psi$of blood- if decrease in $\Psi$, nerve impulses are sent along osmoreceptors to posterior pituitary gland to release ADH
  • ADH enters blood to travel throughout body
  • causes kidneys to reabsorb more water, reducing water loss in urine
• how is water reabsorbed in the kidneys

  by osmosis, from filtrate in the nephron
• effect of ADH on kidneys

  • ADH causes luminal membranes of collecting duct to become more permeable to water
  • done by causing an increase in aquaporins
  • as filtrate travels along collecting duct + aquaporins into blood plasma of medulla, it loses water and becomes more concentrated
  • small volume of concentrated urine is produced
• how does ADH cause an increase in aquaporins

  • collecting duct cells contain vesicles - membranes of vesicles contain aquaporins
  • ADH molecules bind to receptor proteins, activating signalling cascade that leads to phosphorylation of aquaporins
  • aquaporins activated, causing vesicles to fuse with luminal membranes of collecting duct cells
  • membanes permeability to water now increases
• what happens if $\Psi$ of blood is too high?


  • osmoreceptors in the hypothalamus not stimulated
  • no nerve impulses are sent to the posterior pituitary gland -no ADH released
  • aquaporins are moved out of the luminal membranes of the collecting duct cells
  • collecting duct cells are no longer permeable to water
  • filtrate flows along collecting duct but loses no water and is very dilute
  • a large volume of dilute urine is produced
• kidney failure

  if kidneys fail:

  • urea + salts + toxins are retained, not excreted
  • less blood filtered by glomerulus, so toxins build up in the blood
  • concentration of ions + charged compounds in the blood not maintained
• why is the balance of electrolytes important?

  • excess $K^+$ can lead to abdominal cramps + muscle weakness - frequency of impulses from SAN in heart may decrease leading to cardiac arrest
  • $Na^+$ needed for neuromuscular function + acid/base reactions - a build up can cause disorientation + general weakness
• treatments for kidney failure

  • kidney transplant
  • dialysis - substances removed from blood by diffusion via dialysis membrane
• haemodialysis

  • partially permeable membrane separates patient's blood and dialysis fluid
  • fluid contains substances needed in the blood e.g glucose + $Na^+$ in concentration similar to normal level in blood
  • no conc gradient for glucose exists - no net movement
  • salt movement only occurs if there is an imbalance
  • fluid continually refreshed to maintain gradient
  • fluid contains no urea - urea diffuses down conc gradient into fluid to be disposed of
  • fluid and blood flow in opposite directions
  • heparin added to prevent blood clots